Various electro-optical systems have been developed for reading optical indicia, such as bar codes. A bar code is a coded pattern of graphical indicia comprised of a series of bars and spaces of varying widths, the bars and spaces having differing light reflecting characteristics. Some of the more popular bar code symbologies include: Uniform Product Code (UPC), typically used in retail stores sales; Code 39, primarily used in inventory tracking; and Postnet, which is used for encoding zip codes for U.S. mail. Systems that read and decode bar codes employing charged coupled device (CCD) or complementary metal oxide semiconductor (CMOS) based imaging systems are typically referred to hereinafter as imagining systems, imaging-based bar code readers or bar code scanners.
Imaging-based bar code reader systems electro-optically transform the graphic indicia into electrical signals, which are decoded into alphanumerical characters that are intended to be descriptive of the article or some characteristic thereof. The characters are then typically represented in digital form and utilized as an input to a data processing system for various end-user applications such as point-of-sale processing, inventory control and the like.
Imaging-based bar code reader systems that include CCD, CMOS, or other imaging configurations comprise a plurality of photosensitive elements (photosensors) or pixels typically aligned in an array pattern that could include a number of arrays. The imaging-based bar code reader systems employ light emitting diodes (LEDs) or other light sources for illuminating a target object, e.g., a target bar code. Light reflected from the target bar code is focused through a lens of the imaging system onto the pixel array. As a result, the focusing lens generates an image from its field of view (FOV) that is projected onto the pixel array. Periodically, the pixels of the array are sequentially read out creating an analog signal representative of a captured image frame. The analog signal is amplified by a gain factor, by for example an operational amplifier or microprocessor. The amplified analog signal is digitized by an analog-to-digital converter. Decoding circuitry of the imaging system processes the digitized signals representative of the captured image frame and attempts to decode the imaged bar code.
As mentioned above, imaging-based bar code readers typically employ an illumination system to flood a target object with illumination from a light source such as an LED in the reader. Light from the light source or LED is reflected from the target object. The reflected light is then focused through a lens of the imaging system onto the pixel array, the target object being within a field of view of the lens. It is not uncommon for a single reader to employ multiple LED sources or a bank or cluster of LEDs in order to achieve the amount of illumination necessary for a successful reflection from the target object. Such configurations are often costly to manufacture, and also undesirably, demand increased power requirements, thus reducing battery life on portable readers. In addition, multiple LED sources may create non-uniform illumination with hot spots in the image, i.e. local areas of bright light
What is needed is a cost effective imaging-based bar code reader system capable of generating an illumination pattern having a sufficient lighting to produce a successful image from the target object, while minimizing power requirements, maximizing the systems efficiency.